basic darkval for subcubes

graph.cpp

@@ -3781,6 +3781,10 @@ EX int get_darkval(cell *c, int d) {
   const int darkval_kite[12] = {0, 2, 0, 2, 4, 4, 6, 6, 6, 6, 6, 6};
   const int darkval_nil[8] = {6,6,0,3,6,6,0,3};
   const int darkval_nih[11] = {0,2,0,2,4,6,6,6,6,6,6};
+  if(among(variation, eVariation::dual_subcubes, eVariation::bch)) {
+    int v = reg3::get_face_vertices(c, d);
+    return v-3;
+    }
   if(sphere) return darkval_s12[d];
   if(euclid && S7 == 6) return darkval_e6[d];
   if(euclid && S7 == 12) return darkval_e12[d];